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Date: Sun, 23 Nov 2008 11:56:21 +0100 (CET)
From: Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>
To: "Marc G. Fournier" <freebsd@hub.org>
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Cc: freebsd-questions@freebsd.org
Subject: Re: auto-addm new tap device to existing bridge ...
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> Is this possible?
>
> I'm using qemu, and when I start it up, it auto-create a tap device if one
> isn't available ... but, having that tap device not attached to the bridge that
> does exist doesn't help much ... if there some flag I can set, or sysctl value,
> that will have the new tap device attach itself to an existing bridge device?

please create bridges and tap's at startup and use qemu option to 
explicitly give the tap name that's what i do

rc.conf:
cloned_interfaces="tap0 tap1 tap2 bridge0"
ifconfig_bridge0="10.255.245.1/24 up"

in rc.local:

/sbin/ifconfig bridge0 addm tap0 addm tap1 addm tap2


and when using qemu

-net nic -net tap,ifname=tap0,script=no (or tap1, tap2)